When loading a root certificate for a security deployment, one Mac OS X 10.6 Snow Leopard box was unfortunately tossing the 1000013 error code.
Per MacErrors.h, that's the code assigned to a a permissions error:
kPOSIXErrorEACCES = 100013L, /* Permission denied */
If you should receive this 100013 error code while trying to load a certificate using Keychain Access.app, then the usual trigger is some sort of permissions error.
As for repair options, you can run Disk Utility and repair the permissions on your disk, and try loading the certificate again.
Alternatively, you can often avoid the repair and override the permissions problem and install the certificate directly by launching Terminal.app and issuing the following command:sudo security import </Path/To/Certificate.pem> -k /Library/Keychains/System.keychain
This command might be preferable when you're not in a position to repair the disk permissions on the target disk.
This security command is also handy when you're scripting the root certificate load.